MODELING AND ANALYSIS OF CVD PROCESSES IN POROUS-MEDIA FOR CERAMIC COMPOSITE PREPARATION

A continuum phenomenological model is presented to describe chemical vapour deposition (CVD) of solid product inside porous substrate media for the preparation of reinforced ceramic-matrix composites [by the chemical vapour infiltration (CVI) process] and ceramic membrane composites (by a modified CVD process). The chemical reaction, intrapore diffusion, non-isobaric viscous flow and variation of substrate pore geometry during deposition are considered in the model which is readily solved by the orthogonal collocation numerical technique. Simulated deposition profiles across substrate are given to examine the effects of the reaction mechanism, reaction and diffusion rate, substrate pore dimension, deposition temperature, bulk phase reactant concentration, intrapore diffusivity of reactants and pressure drop on the deposition results of a one-dimensional isothermal forced-flow CVI process and a modified non-isobaric CVD process for ceramic composite preparation. The theoretical analysis provides a better insight of the CVD processes in porous media and is useful in explaining experimental findings and guiding the selection of optimum process conditions for the CVD preparation of ceramic composites.